Parallel shaft transmissions with such a constitution have had many uses such as in automobile transmissions. For example, parallel shaft automatic transmissions are disclosed in Japanese Patent Laid-open No. 57-103955(A) and Japanese Patent Laid-open No. 4-331852(A).
However, these parallel shaft transmissions have constitutions wherein a plurality of gear trains for engaging with each other are disposed parallel to the shafts and between the parallel shafts, and the gear train for effecting power transmission is selected by the connection and disconnection of a plurality of clutches disposed adjacent to the gears constituting each gear train. Therefore, the tendency is for the axial dimension of the transmission to increase; this tendency increases as the number of speeds increases. Particularly in automobile transmissions, the number of hydraulic clutches corresponds to the number of speeds, but hydraulic clutches structurally have relatively large axial and radial dimensions, and it is easy for the transmission to become large.
Also, in recent years, automobile transmissions have tended to have a greater number of speeds because of the demands for drivability and improvements in fuel consumption. Three speed transmissions changed to four speed transmissions; once five speed transmissions came into use, six speed transmissions were requested because of further demands for drivability and improvements in fuel consumption. This is consequently a situation where it is easy for transmissions to become larger and larger. Likewise, as the transmissions came to have larger numbers of speeds, the number of clutches increased and the increased size and weight of transmissions and consequent increases in manufacturing costs have become problematic.